1. Field of the Invention
The present invention pertains to a grip for tensile and similar testing, with at least one jaw which includes a swivel face with one or two degrees of freedom in order to accommodate variations in the surface of the test specimen while maintaining a secure grip on the test specimen.
2. Description of the Prior Art
The art of tensile testing, along with similar methods of testing such as shear testing and compression testing, is well-developed. An example of a device used in such testing is a video extensometer as disclosed in U.S. Pat. No. 7,047,819 entitled “Testing of Samples”, issued on May 23, 2006 and patent application Ser. No. 11/410,686 entitled “Testing of Samples”, filed on Apr. 24, 2006. These devices generate stress/strain curves for a tested sample. In order to measure the strain, which is calculated through the percentage of linear deformation of the sample under testing, video targets, such as two dots, are placed on the sample. The extensometer uses video methods to determine the change in distance between the targets during testing, thereby calculating the strain. As the cross-sectional area of the target is calculated prior to testing and the force applied to the target is recorded, the resulting stress can be correlated with the strain to generate a traditional stress/strain curve.
Devices known as “wedge action grips” are commonly used to grip the samples in such testing machines. These grips utilize the taper angle of the body and jaw face to apply a parallel side acting force to grip the specimen. In most grips, a force is applied to the base of the jaw face pushing it vertically with respect to the body, so that the taper or wedge causes the jaw to close horizontally. Force can be manually applied through a screw thread, a lever arm, a gear or similar devices, or through powered means such as electric, pneumatic or hydraulic power through a piston or a motor. Typically, in all methods which move the jaw face with respect to the grip body, the surface of the jaw faces are maintained in parallel configuration to each other by the grip body. The jaw faces therefore are assumed to engage the flat, parallel faces of the test specimen. However, typically, neither the jaw faces nor the sample faces are exactly parallel. This is due to the manufacturing tolerances of the body angles and jaw faces of the grip, deflection of the grip body under stress changing the angle of the gripping surface against the specimen and the quality of the test specimen.
This lack of parallelism between jaw face and the surface of the specimen results in uneven engagement of the serration of the jaws into the specimen surface. For some materials, this is acceptable but for others, this will result in specimen slippage. The existing solution to this has been to utilize high powered grips that will exert sufficient clamping force to overcome the lack of parallelism. Similarly, in the prior art grip design, with specimens of uneven surface, the fixed face of the jaws may create bending moments causing uneven stresses on the specimen which can, under many circumstances, distort the test results.
Additionally, the prior art jaws use a gripping surface of a fixed material. That is, to change the gripping surface, the entire jaw must be changed. This adds to the complexity of operating the device. Additionally, there may be some selected circumstances, such as with a laminated or layered sample, wherein it may be desired to have opposed jaws with two different gripping surfaces to accommodate two different exposed surface materials.